Positron emission tomography (PET) is a well-established molecular imaging modality. Current clinical PET scanners are large, bulky devices that are placed in dedicated imaging rooms and require the subject to be brought to the imager. With very few exceptions, PET scanners are typically limited to imaging subjects in a supine or prone position. They are also typically combined with CT scanners, which are not easily amenable to other than horizontal imaging geometries. Functional magnetic resonance imaging (fMRI) cannot be used for functional brain imaging of upright subjects, because present day upright MRI scanners do not provide strong enough magnetic field for functional imaging. In addition, MRI requires that the subject be immobile during the scan.